1. Field of the Invention
The present invention generally relates to an image formation apparatus, and especially relates to density control of the image formation apparatus.
2. Description of the Related Art
In image formation apparatuses such as a printer, a copying machine, and a facsimile apparatus, a latent image is electrostatically formed on a photoconductor serving as an image supporting object, toner is applied to the latent image to obtain a toner image, the toner image is transferred to a transfer medium such as paper, and the transferred toner image is fixed for output.
For developing the latent image into the toner image, a two-component developer, wherein the toner is mixed with a carrier, is often used.
When developing, in order to obtain uniform density of images to be formed on the photoconductor, image density control for controlling the supply of the toner is performed such as described below.
According to a method (first conventional method), a “patch image” is formed on the photoconductor, the density of the patch image is measured by an image density sensor (P sensor), and toner supply is controlled according to a result of the image density measurement. Here, the patch image is a reference image having a reference density for the density measurement.
According to another method (second conventional method), toner density (content of the toner in the developer) is measured. Especially when the developer is made of the toner and a magnetic carrier, the toner density (content of the toner) is measured in terms of the permeability of the developer by a permeability sensor (T sensor). The supply control of the toner is performed according to the toner density measurement (for example, Patent Reference 1).
The first conventional method has a problem in that the patch image cannot be formed during image formation if a transfer belt and a transfer roller are used for image transfer wherein a transfer surface contacts the photoconductor. Further, when printing is continuously performed, the image density control using the patch image cannot be performed in the meantime; accordingly, even if the image density fluctuates, the image density cannot be corrected.
The second conventional method has a problem in that the image density control cannot follow a change of “development capability”. If the development capability changes density of a resulting image changes. The change of the development capability can occur with degradation of the developer, a change of the environment, and a difference of development conditions of developers and apparatuses.
Then, a density control method combining the first and the second conventional methods is proposed as disclosed by Patent Reference 1 so that the problem of one is compensated for by the other.
According to the combined method, the toner density is controlled so that a toner density value (Vt) measured by the T sensor may be in agreement with a toner density reference value (Vtref). The toner density reference value (Vtref) is obtained by “a previous toner density value (Vt)−a compensation value (ΔVtref)”. Here, the predetermined compensation value ΔVtref is obtained using an image density value (Vp) of the patch image, which Vp is equal to Vsp/Vsg (Vsp=detected electric potential, and Vsg=electric potential corresponding to background optical density). Then, the toner supply is controlled so that the Vt becomes equal to the toner density reference value Vtref.
With reference to FIG. 6 that is a flowchart representing the process described above, whether the number of sheets processed since the last P sensor detection reaches a predetermined value (e.g., 10 as shown in FIG. 6) is determined every time the copying process is completed. If the determination is affirmative (i.e., if the number of processed sheets reaches the predetermined value), the patch image is generated, and the P sensor detects the density (Vp=Vsp/Vsg) of the patch image.
In addition to the image density detection, the toner density is detected. Specifically, the T sensor detects a present Vt; the compensation value ΔVtref is determined based on the previous Vt, the present Vt, and Vsp/Vsg; and then, the Vtref is updated and stored.
The updated Vtref (to serve as the target value of the T sensor), and the difference (i.e., Vt−updated Vtref) for every one copy are obtained as described above; then, toner supply conditions for every copy (image formation) are determined.
However, when the copying process is performed for a greater number of sheets than the predetermined number, since the patch image cannot be formed until the process is completed, the image density control cannot be performed, so the toner density control is carried out based on the previous Vtref.
As described above, the problem of the second conventional method is compensated for by the P sensor, which problem is in that the image density control cannot follow the change of development capacity due to degradation of the developer, the change of the environment, and the difference of the development conditions of developers and apparatuses; the problem of the first method is compensated for by the T sensor so that the image density may be uniform, which problem is in that the patch image cannot be formed during image formation if the transfer belt and the transfer roller are used for image transfer wherein the transfer surface contacts to the photoconductor (e.g., Patent Reference 1).
[Patent Reference 1] JPA 2001-125365